Stems erect, 38–80(–90) cm, glabrous; branches borne on distal part of stem, at least slightly exceeding infructes­cence. Leaves: petiole 0–3(–5) mm, glabrous; blades somewhat heteromorphic or all linear; stem leaf blades linear to ligulate or very narrowly lanceolate to very narrowly elliptic, (5.5–)7–12(–14) cm × (2.5–)3–6(–8) mm, margins entire, sometimes slightly revolute, sparsely ciliate, apex narrowly acute, surfaces glabrous; branch leaf blades linear, (4.8–)5.5–9(–10) cm × (1–)1.5–2.6(–3.1) mm. Flowers: sepals subulate to narrowly deltate, often weak and curved, (3–)4.5–6.5(–7) mm, glabrous; corolla tube blue (to whitish), 16–21.5 mm, lobes bluish to white, turning blue with age (pinkish), 9.5–14(–16) mm, outer surface of corolla glabrous. Follicles erect, 4–12(–15) cm × 3.1–4(–4.5) mm, apex acuminate, glabrous. Seeds 6.4–11 × 2–3 mm.

Flowering spring (rarely summer); fruiting summer. Woodlands, hillsides, canyon bottoms and gullies, open ground, often near water; of conservation concern; 800–1400 m; Ariz.; Mexico (Sonora).

Amsonia grandiflora is primarily restricted to Pima and Santa Cruz counties and rarely extends to Maricopa County and adjacent regions of Mexico. In Mexico, it is largely or entirely restricted to the state of Sonora; it reportedly rarely occurs in Durango, but no specimen was seen. It may be distinguished from A. palmeri by its larger flowers. Fruiting specimens of A. grandiflora will typically have somewhat shorter leaves than narrow-leaved individuals of A. palmeri, and the sepals are on average longer.